The production of C.sub.2 -C.sub.5 olefins from liquified petroleum gas (LPG) by catalytic dehydrogenation involves the use of cycled catalytic reactors. Reactors containing a catalytic bed in the form of cylindrical pellets of chromic oxide on active alumina support are typically used to dehydrogenate LPG feed materials which are then compressed and separated to yield desired products including propylene, isobutylene, other butenes and pentenes, light hydrocarbon gases, and hydrogen gas. Typical of such a reaction system is the Catofin.TM./Catadiene.TM. system. The general reaction catalyzed within the system is the dehydrogenation of alkane hydrocarbons to alkenes. EQU C.sub.n H.sub.(2n+2) .fwdarw.C.sub.n H.sub.2n +H.sub.2 (I)
This dehydrogenation of LPG to produce olefins is a highly endothermic reaction. For example, approximately 1,000 BTUs must be supplied per pound of product double bond produced for C.sub.4 compounds and about 1,300 BTUs must be supplied per pound of product double bond for the propane-to-propylene reaction. The needed reaction heat is primarily supplied by heat stored in the catalyst bed which is heated in an off-stream part of the cycle.
The Catofin.TM. and Catadiene.TM. processes are described in numerous publications including Craig, R. G. et al, Chemical Engineering Progress, February, 1979 pp. 62-65 and Gussow, S. et al, Oil & Gas Journal, Dec. 8, 1980 pp. 96-101.
The prior art methods of reactor reheating include the combustion of coke deposited on the catalyst during the dehydrogenation reaction. This deposited coke was burned with preheated air to heat the catalyst bed to the desired temperature. Additionally, fuel gas may be burned in the reactor to provide additional energy. Another source of the heat required by the dehydrogenation reaction is the sensible heat of air which is pumped through the reactor.
Once reheated to the desired temperature, a reactor was cycled for a short time with hydrogen to reduce the catalyst and placed on-stream where the endothermic dehydrogenation reaction absorbed the energy, decreasing reactor temperature during the on-stream period.
This reactor cycle included a steam purge of the reactor to remove hydrocarbon materials, evacuation of the reactor, reheating with air or air and fuel gas mixture concurrently with coke combustion to heat the reactor to the desired temperature, post-reheat evacuation, exposure to a reducing gas, and resumption of dehydrogenation processing. In order to provide continuous major process stream flows, at least two reactors are needed. Commercially, systems of three to eight reactors are commonly employed. A central timing device programs the cycle sequence for reactor reheat, purge, and operation phases.
Prior art methods for reheating the catalytic reactor have several demonstrated shortcomings. When operated with higher fractions of non-diolefin-forming hydrocarbons, the amount of coke deposited on the catalyst bed during the on-stream period is generally insufficient to provide needed energy during the reheat cycle.
Furthermore, newer, more efficient dehydrogenation catalysts yield less coke than older catalyst types for equal amounts of product olefin produced. Thus, systems using these new catalysts require alternative sources of energy during the reheat cycle. This additional energy must be supplied by heating air to very high temperatures or by burning light hydrocarbon fuel gases in the reactor. Both alternatives are undesirable.
Whether additional heating of the recycle air stream or fuel gas combustion in the reactor is employed, excessive temperatures may be encountered at the top of the bed, leading to catalyst deactivation. In addition, uneven catalyst heating leads to cooler spots as well as areas of deactivated catalyst, both of which degrade process performance.
Reheating of the catalyst bed by heated air, burning of coke, or burning of fuel gas also carries with it the added disadvantage of introducing a thermal gradient to the bed. During reheat, the top of the bed will heat first and most rapidly, while the lower regions will heat slowly. Because catalyst deactivation is a primary limit on the reheating cycle, the reheat must be stopped when the top of the bed reaches maximum allowable temperatures. At these conditions, the bottom of the catalyst bed is generally far below its optimum temperature.
When placed back on-stream, this thermal gradient causes enhanced activity at the top of the bed and reduced efficiency toward the bottom. As a result, greater amounts of coke are deposited at the top, which in turn leads to a perpetuation of the undesirable thermal gradient during subsequent reheating cycles.